1. Field of the Disclosure
The present disclosure relates to a device and method for performing an initial orientation calibration for an inertial measurement unit.
2. Description of the Related Art
An Inertial Measurement Unit (IMU) is an electronic sensor suite, generally comprised of accelerometers, gyroscopes, and magnetometers, which is used to measure the motion relative to an initial position and orientation. Among other things, IMUs are increasingly utilized to perform golf swing analysis, where the IMU measures the swing motion of a golf club relative to an initial position and orientation. However, an IMU is intrinsically unable to quantify the motion in terms of absolute position and orientation. For example, in a case where an IMU is utilized to perform a swing analysis in golf, the IMU can report that the club face closed 2° over the course of the swing, but it cannot determine whether the face was closed or open at the start of the swing. Thus, meaningful results often depend on a priori knowledge of the initial orientation of an IMU to its surroundings.
In a golf application, there are typically two necessary transformations that must be known when determining an initial IMU orientation in order to perform golf swing analysis. The first is a transformation from the initial IMU coordinate system to a world set of coordinates—usually those that describe the target line, vertical, and other relevant directions for the golf swing. The second is a transformation from the IMU to a coordinate system fixed in the golf club.
Methods for establishing the club transformation currently rely on some pre-defined protocol, rather than a direct measurement. For example, current methods assume that an IMU is placed at a particular position on the club shaft (e.g., the 3 o'clock position), and at a specific distance from the club face. However, any deviation from this pre-defined position will cause corresponding inaccuracies in the results. Similar methods rely on the golfer aligning the IMU device or/and the face normal to within a degree of some reference, which introduces user error. Consequently, the accuracy of many output parameters is limited not by the IMU itself, but rather by the uncertainty in the club face orientation.
Another method assumes that the club face points in a direction perpendicular to both the club shaft and gravity at the point of address (i.e., the point at which a golfer is in position to start a swing). This method is accurate only to the extent that the golfer addresses the ball with zero forward or backwards shaft lean, and with a perfectly flat lie. The latter consideration should be explicitly addressed: most golfers consider the club face square if the leading edge is perpendicular to the target line; however, if the club does not lie flat on the ground (many golfers address the ball with the toe up), the face normal does not point in the same direction, and thus is not square. Consequently, inaccuracies are introduced by utilizing this technique.
Another method correlates the club face normal with the direction of the initial motion of the club during takeaway (i.e., the initial movement of the golf club away from a golf ball during the swing). This is accurate only if two conditions are met: (1) the club face normal is square with the target line, and (2) the club moves along the target line during takeaway. These conditions are rarely met for the average golfer, which results in inaccurate swing analysis results.
A different class of methods require some procedure before the swing starts. These typically involve the use of a magnetometer. For example, one procedure requires the golf club to be held with zero shaft lean. When positioned correctly, it is tapped on the ground, which signals the electronics to reference the indicated target line (the direction perpendicular to the club shaft and gravity) to a particular compass direction. Alternately, the club can be pointed down the target line, which is again recorded with respect to the local magnetic field. Both these methods are quite susceptible to user error, and both assume the face normal was square with the target line at address, which is generally not true.